Magnesium is the lightest known structural metal, approximately ⅕ the density of steel, ½ the density of titanium, and ⅔ the density of aluminum. Magnesium alloys represent potential weight savings and therefore fuel savings across the entire transportation industry. Predominant texture (also called “basal texture”, and hereinafter called “texture”) in magnesium alloys is an important factor limiting the formability of magnesium alloys. Certain cost barriers have heretofore precluded widespread utilization of magnesium and magnesium alloys. Two cost factors addressed in recent initiatives include (1) elimination of rare earth alloying elements and (2) lowering the forming temperature.
Magnesium alloys containing rare earth elements have been developed that have improved formability over conventional magnesium alloys, and allow forming to take place at temperatures below 200° C. The 200° C. threshold is desirable for economic reasons and is the approximate upper temperature limit where conventional oil based lubricants can be used for die lubrication during forming. The removal of the die lubricants with solvents in automated machinery falls within the normal parameters associated with low cost forming operations. Forming operations that are required to take place above 200° C. use solid lubricants where post forming lubricant removal is by mechanical means, followed by surface buffing to achieve acceptable surface finishes. The labor input and processing complexities associated with removal of solid lubricants after forming adds undue cost and limits magnesium's potential use in high volume complex geometry automotive panels. The rare earth containing alloys that allow forming below 200° C. however are more costly and could become scarce due to the supply of rare earth metals. Therefore, initiatives for magnesium sheet in automotive application have been focused on achieving equivalent or superior formability at 200° C. and below, without rare earth additions.
Conventional non rare earth containing magnesium and magnesium alloy sheet require forming temperatures above 300° C., due to the presence of an undesirable strong hexagonal close packed crystalline texture, inherent in the sheet after conventional processing that includes symmetric rolling. Such a texture is the reason metal sheet is insufficiently ductile for forming into useful shapes below 200° C. Therefore a need exists for processing magnesium sheet by shear rolling in the range of 180-250° C. to form a disrupted texture, and avoid formation of an undesirable, strong hexagonal close packed texture, thereby producing desired forming characteristics at 200° C. and below.
The skilled artisan will find helpful information regarding the use of asymmetric rolling to decrease the strong texture of Mg in the following publication:    Benoît Beausir, et al., “Analysis of microstructure and texture evolution in pure magnesium during symmetric and asymmetric rolling”, Acta Materialia 57 (2009) 5061-5077.
The skilled artisan will find helpful information regarding the use of asymmetric rolling to decrease the strong basal texture of Mg—Al—Zn alloy in the following publications:    Xinsheng Huang, et al., “Microstructure and texture of Mg—Al—Zn alloy processed by differential speed rolling”, Journal of Alloys and Compounds, 457 (2008), 408-412.    W. J. Kim et al., “Microstructure and mechanical properties of Mg—Al—Zn alloy sheets severely deformed by asymmetrical rolling”, Scripta Materialia 56 (2007) 309-312.